Respiratory disease is a common and significant problem in both the United States and throughout the world. Obstructions generally make expiration slower and/or more difficult as more air is exhaled from the lungs or during the latter part of expiration. The obstruction can stem from the constriction of the airways during respiration, inflammation and edema of the walls of the terminal bronchi, or narrowing of the trachea or the throat. Types of respiratory disease include diseases of the lung, bronchial tubes, pleural cavity, upper respiratory tract, trachea, and of the nerves and muscles of breathing.
An important step in monitoring for and managing such diseases, as well as less severe respiratory conditions, involves measuring flow rate and/or the volume of air moving into and out of the patient's lungs. Methods of measuring such flow data include several standard spirometry methods. One illustrative spirometry method includes covering a patient's mouth and nose and channeling his/her breathing through a tube to a meter that measures the speed of sound as it propagates through the moving air. The effect that the moving air has on sound wave speed is indicative of an airflow rate. Another method involves a pneumotachometer in which a grid or mesh provides a small amount of resistance to the air as it flows through the grid. The pressure difference across the grid is measured, and that measurement provides information indicative of an airflow rate.
Methods of measuring such volume data include enclosing a patient entirely in a tank, with only his/her head extending out of the tank. An airtight seal is then provided around the neck and a gauge is incorporated into the tank for measuring displaced air as the patient's lungs expand and contract. The displaced air measurements are indicative of inhaled/exhaled volume. Another method of measuring the volume of air moving into and out of a patient's lungs involves covering a patient's mouth and nose and channeling his/her breathing through a tube to a bellows or piston assembly. The air can cause the bellows or piston to mechanically displace, and the displacement can be measured and correlated to the inhaled/exhaled volume. Additionally, VoluSense, the assignee of the present invention, has developed one such method, which is disclosed in VoluSense's U.S. Pat. Nos. 6,374,667; 6,945,941; and 7,390,307. These patents are hereby incorporated by reference herein in their entirety.
Another important step in monitoring for and managing respiratory conditions involves analyzing and managing the collected flow data and volume data. As is discussed herein, an important tool for doing so is a flow/volume loop, which is a graph constructed from simultaneous recordings of exhaled volume and airway flow. Flow/volume loops can contain useful information related to a patient's respiratory condition. However, existing methods of analyzing and managing flow/volume loop information have a variety of drawbacks, which limit the quality and quantity of useful information that can be extracted from a flow/volume loop.